Added minimal_plastic_spring_extruder

See: http://www.thingiverse.com/thing:542498

Inspired by http://www.thingiverse.com/thing:411462, but designed from
scratch in OpenSCAD, using http://www.thingiverse.com/thing:46749.
Added spring for tensioning without any additional hardware. For Mk8
drive gear, 3mm filament and 624ZZ bearing. Currently untested, so
dimensions and spring constant likely wrong.
To do:
- test spring
- add finger pull
- add filament guides
- add dust box
- add hot end/bowden mount
- add fan mount(s)
- add x carriage